I have known many answers but none seems to fit my situation.
On Guam, we have a busy commercial airport, an air force base, and a navy air base... and NO contrails. NONE. Where do contrails come from.
"Why, they come from the back of planes Mk." No, I don't want that answer.

Contrails happen when the exhaust from jetplanes become saturated in the air. This happens if the vapor pressure and temperature of the exhaust as it leaves the plane and and while it comes to equilibrium with the atmospheric vapor pressure and temperature meets the necessary conditions for saturation.

Contrails happen when the exhaust from jetplanes become saturated in the air. This happens if the vapor pressure and temperature of the exhaust as it leaves the plane and and while it comes to equilibrium with the atmospheric vapor pressure and temperature meets the necessary conditions for saturation.

Since Guam is fairly close to the equator, its atmospheric temperature at jet plane height might be great enough such that there is not enough vapor pressure to saturate in the air.

It's the moisture in the combustion gases that condenses. I have seen contrails overhead since my area gets a lot of air traffic from Boston and the NE airports, and international flights into NY. However, contrails do not always form. That is a matter of air temperature as Jelfish indicated, in which case the combustion gases dissipate before condensing into the appropriate size droplets. I have been in commercial jets and seen other commercial jets flying underneath or overhead in the opposite direction (which is pretty amazing to see), and there were no contrails. That's more the case in the summer than during the cooler fall, winter and spring periods.

Typcial altitudes for big commerical jet aircraft are about 8000-12300 m (26000 - 40000 ft) over land, with slightly higher altitudes over the ocean (10000-14000 m) for transcontinental flights. Short range flights tend to be at the lower altitudes to minimize congestion of the airspace.